As high strengthening of automobile bodies and structural materials is required in view of improvements in fuel efficiency and stability resulting from the lightening of automobiles, many kinds of high strength steels for automobiles have been developed. However, since high strengthening decreases ductility in most steel sheets, many types of steel sheets resultantly have many limitations in the processing thereof to form parts. To solve the problem of a decrease in ductility due to the high strengthening of steel sheets, many studies have been undertaken, and as a result of the studies, austenite-based high-manganese steel (see JP1992-259325, WO93/013233, WO99/001585, WO02/101109, and the like) has been proposed in which 5-35% by weight of manganese is contained in steel to induce twin boundary defects deformation during plastic deformation of steel, thereby remarkably improving ductility.
Then, the high-manganese steel has a problem in that the coating adhesion of the hot-dip galvanized steel may be relatively poor. That is, since hot-dipped galvanizing of a steel sheet improves corrosion resistance, weldability and paint coatability, a majority of steel sheets for automobiles are hot-dip galvanized. Then, hot-dip galvanized steel sheets which use high-manganese steel as a material to be galvanized are annealed in a nitrogen atmosphere containing hydrogen for the securing of desired material qualities and surface activation (reduction). Such an atmosphere is a reducing atmosphere with respect to matrix iron (Fe) that is a material to be galvanized but acts as an oxidizing atmosphere with respect to elements which are easily oxidizable, such as manganese (Mn), silicon (Si), aluminum (Al), and the like, in high-manganese steel. Therefore, when high-manganese steel containing a large amount of Al, Si, and the like, as well as Mn being annealed for recrystallization in such an atmosphere, alloy elements are selectively oxidized by a trace of moisture or oxygen contained in the atmosphere to form a Mn, Al, Si surface oxide layer on a surface of the matrix material (to be galvanized). Thus, when high-manganese steel containing a large amount of Al, Si, and the like is used as a material to be galvanized, coating failures occurs due to a surface oxide formed during an annealing process that is a pre-coating process, or even in the case that a galvanized layer is formed, the galvanized layer may be delaminated during a processing process.
To date, as publicly disclosed art related to preventing coating failures of a high-manganese and hot-dip galvanized steel sheet, there are provided 1) a plating method (Korean Patent Application Publication No. 2007-0067950) in which silicon (Si) is added to form a thin Si oxide film on a surface, thus suppressing the formation of an Mn oxide, 2) a method (Korean Patent Application Publication No. 2007-0107138) in which a 50 nm to 1000 nm of aluminum layer is deposited by a physical vapor deposition (PVD) followed by annealing to prevent the formation of an Mn oxide, and the like.
However, since in the case of method 1, Si has a higher oxidation potential than Mn to form a stable film type oxide, it is impossible to improve wettability with molten zinc. Also, since method 2 requires a vacuum deposition process followed by annealing for galvanizing, Al, a material to be galvanized, is easily oxidizable, and the deposited Al forms an oxide having poor wettability due to moisture or oxygen contained in the annealing atmosphere, method 2 may rather deteriorate galvanizability.
As described above, in the existing publicly disclosed art, when high-manganese steel containing a large amount of Mn is used as a plating material, since thick Mn, Al, Si oxides or composite oxides thereof formed during the annealing cause coating failures, or a galvanized layer is formed, the galvanized layer simply covers a thin oxide layer without an interfacial inhibition layer at an interface between the plating layer iron, coating delaminations in which the galvanized layer is separated from matrix iron during a processing process may occur.